Electric cables used in industrial facilities, like, for example, onshore and offshore plants, or in public infrastructures, like, for example, underground lines, require high fire resistance properties to maintain operation of the facilities or infrastructures for a certain period of time to allow evacuation of the people before the fire breaks out.
Due to the increasing amount of equipment running on medium and high voltage in the abovementioned industrial facilities or public infrastructures, there is the need of having fire resistant medium and high voltage electric cables.
Typically, medium (MV) and high voltage (HV) electric cables comprise a core and an outer sheath arranged in a radially outer position with respect to the core. The core comprises a conductor, an insulating system arranged in a radially outer position with respect to the conductor and a conductive screen arranged in a radially outer position with respect to the insulating system. The insulating system typically includes a first semiconductive layer arranged in contact with the conductor, an insulating layer arranged in a radially outer position with respect to the first semiconductive layer, a second semiconductive layer arranged in a radially outer position with respect to the insulating layer.
In specific cases, such as, for example, in off-shore plants, the electric cables also include a metallic armor radially interposed between the conductive screen and the outer sheath.
In order to provide a medium and high voltage electric cable with fire resistant properties, a fire resistant layer is typically provided in the cable to create a barrier to the fire propagation within the cable.
EP 2413331 relates to a medium voltage fire resistant electric cable comprising a first layer made of an extruded elastomeric ceramics silicone material, and an optional outer layer made of a like material, thereby providing a thermal protective barrier for the innermost layers of the cable comprising copper conductors, a semiconductor material and an insulating material sheath. In particular, the insulating material sheath is coated by an extruded silicone and ceramizing elastomeric layer. In cooperation with the ceramizing elastomeric layer, a second semiconductor material layer is further co-extruded. The cable is further provided with a screen or shield constituted by a copper strip and a thermal insulating layer including a plurality of glass fiber strips covering in turn the copper shield. The layer made of an extruded ceramizing silicone elastomeric material provides both an electrical insulation and a thermal insulation. Another glass yarn strip operates to further increase the cable fire thermal resistance.
JP 05-182532 relates to a fire resistance cable suitable for the high voltage use of 6600V class. The cable core includes an inner semiconductive layer, an insulating layer, an outer semiconductive layer, a metal shield layer and an external sheath. The cable core is surrounded by a layer of mica and cellulose pulp adhering onto a reinforcement layer made of, e.g. glass fiber. This layer is surrounded, in turn, by a foaming fire preventing layer which is foamed to be carbonized by heat. The outer sheath is made of PVC.
WO 2014/081096 and counterpart US 2011/0153968 relate to a fire resistant cable for medium or high voltage comprising a conductor, a first semiconducting layer formed outside of the conductor, a first insulation layer formed outside of the first semiconducting layer, a second semiconducting layer formed outside of the first insulation layer, a shield layer formed outside of the second semiconducting layer and a fire resistant layer provided between the first insulating layer and the second semiconducting layer. The fire resistant layer is formed by winding a mica tape two or more times.